3VING AND STATIONARY ELECTROSTATIC .SYSTEM COMPARED. 201 us is equal to the dSr in the equation (299), but the symbol has t a new meaning. Having already used several times the word leetive", I shall now •— only for the sake of uniformity and without ;aching any further meaning to the words — call dS' the effective iment of volume. A point within dS will also be said to belong the efeetive element dS'. Finally, if the charge $dS of an element dS is divided by the ,gnitude of the effective element dS'} we get the quantity $' that defined by (290). For this reason it is not inappropriate to call gr ) effective density of the charge. It will now be clear that the operations involved in the symbol the right-hand side of the equation (801) may be described in tns relating only to the real system, the denominator r' being the ictive distance between a point of the effective element d$' and point P for which we want to calculate cpf. This potential having n determined, its partial differential coefficients with respect to the ctive coordinates, taken with the signs reversed, will represent components of the vector cT. It is only for moving systems that we have had reason to anguish between the effective coordinates and the ,,truew coordi- es, the effective elements of volume and the ,,true" ones, etc.; as a as w «— 0, we shall have x » #r — x, y «-,//,, «« yf z »« zr »« zt '*=*dS, (/*=*£, etc. Yet, for the very reason, of these equalities, are free also to speak of the effective coordinates, tlio effective gity, etc. in the case 'of a stationary system; only, we munfc not ;et that in this case these quantities are identical with the true rdinates, the true density, etc. Similarly, we may always speak he vector (T, remembering that it ia identical with (I when there .o translation. I have dwelled at some length on these questions of d«nomi- on, because in intricate problems a proper choice of terms ia auch value. That which we have now made enables us to coa- le into few words what was said in the last paragraph about tha sms g and g0, namely: In two electrostatic systemg, tlie one moving the other not, in which the effective density of the electric ge is the same function of the effective coordinates, th« vector d' be the same at corresponding points, and the forces will be re- I to each other in the way expressed by (BOO). 172, Let us now return from this digression, to the hypothesis rhich we have tried to account for tie result of MichelHe»n'» riment, "We can understand the poanibility of the assumed ge of dimensions, if we keep in mind that the form of a body depends on the forces between ita molecules, and that, in path of a ray were mapped out by means of suitably arranged screens